Historically, TD-LTE and WiMAX networks both share similar TDD structure. Due to different framing structure, the switch between downlink and uplink transmission is not synchronized across these systems. Therefore, in case both networks are deployed in frequency channels that are close, a severe coexistence issue is created, either at the base station side, or at the terminal side. This is illustrated in the FIG. 1, where the reception (i.e. on the downlink) of data (called LTE data) transmitted by a first base station to a first user equipment (UE1) using LTE technique is being interfered at the user equipment UE1 side by the transmission (i.e. on the uplink) of data (called WiMax data) by a second user equipment to a second base station using the WiMax techniques.
This can prevent proper deployment of a TD-LTE network in such an environment and thus limit the interest for users.
The main issue of interference arises from the so-called near-far effect, in which a given UE (for example UE1) is interfered by a WiMAX UE (for Example UE2) in close vicinity (the “near”), thus preventing the first UE from properly receiving signal from the base station (which is called eNodeB) (the “far”). Specifically, if WiMAX UE is transmitting (beginning of WiMAX uplink), when TD-LTE UE is still receiving the downlink from the eNodeB (end of TD-LTE downlink), then the interference issue arises because WiMAX signal is received at much higher power than signal received from eNodeB.
To avoid these problems, some solutions have been envisaged.                A first solution consist in an alignment of frame structures: this consists of making changes for example to WiMAX and/or LTE frame structures, to have downlink and uplink portions of the frame as aligned as possible. This is possible because both WiMax and LTE are TDD.        A second solution consist in hardware filtering: this consists in using powerful filters on the user equipment side to cope with the interference generated by the other technology.        
A disadvantage of the first solution is to dramatically limit the throughput compared to the throughput which should be obtained without the solution. This results in a poor user experience. Indeed, compared to the “normal” expected throughput, the real one is reduced by at least 20%, which is not acceptable.
In other words, aligning frame structure between WiMAX and TD-LTE bears a cost in terms of capacity and maximum throughput, either on WiMAX or TD-LTE network (depending on which one pays the price for alignment). This means direct cost impact for the operators, and degraded user experience.
Moreover, this cost is paid by all devices and the entire network, whereas there may be not so many actual cases where interference arises.
A disadvantage of the second solution is its cost: adding expensive filters results in an increase of the user equipment. To achieve good rejection, it is required to have three filters (two on the reception—Rx-path, and one on the transmission—Tx-path). Each individual filter has a cost estimated to be $0.5 to $1, and thus total added cost is quite high. Furthermore, the existing user equipments, which do not comprise filters, still have difficulties receiving data.
Second, filtering is not possible if the operator wishes to have flexibility on the bands it wants to use with for instance TD-LTE or WiMAX (for instance an operator that would deploy both technologies).
Third, if the band of the TD-LTE operator is non-contiguous, then hardware filtering is nearly impossible to achieve.